Nanowires and Method for the Production there of

ABSTRACT

The invention concerns the production of segmented nanowires and components having said segmented nanowires. 
     For the production of the nanowire structural element, a template based process is used preferably, wherein the electrochemical deposition of the nanowires in nanopores is carried out. In this manner, numerous nanowires are created in the template foil. 
     For the electrochemical deposition of the nanowires, a reversed pulse procedure with an alternating sequence consisting of cathodic deposition pulses and anodic counter-pulses is carried out. By this means, segmented nanowires can be produced.

FIELD OF THE INVENTION

The invention concerns nanowires with a special structure, a process forproduction of said as well as components made of these nanowires.

BACKGROUND OF THE INVENTION

In “Chemistry in Microstructured Reactors,” Ang. Chem. Int. Ed. 2004,43, 406-466 [: Applied Chemistry, International Edition], K. Jahnisch etal. have demonstrated the advantages that microstructured componentshave in chemical reactions and for analytical purposes. This has led toan increase in the importance that such systems have for chemicalsynthesis and analysis. In comparison to conventional reactors, thesemicrostructures have a large surface area/volume ratio, which has apositive influence on the transference of heat as well as the process ofthe transportation of matter (see also: O. Wörz et al. “Micro-reactors—ANew Efficient Tool for Reactor Development,” Chem. Eng. Technol. 2001,24, 138-142).

Many known reactions have been carried out in microstructure reactors,including many catalytic reactions. For these, it is unimportant whetherthe reactions are liquid phase, gas phase or gas-liquid phase reactions.In order to take advantage of the potential activity of the catalyzer,the catalytic material is integrated in microstructured systems withvarious geometric forms. In the simplest case, the reaction materialused for the construction of the micro-reactor consists in itself of thecatalytically active substance (see also: M. Ficthner, “MicrostructuredRhodium Catalysts for the Partial Oxidation of Methane to Syngas underPressure,” Ind. Eng. Chem. Res. 2001, 40, 3475-3483). This means howeverthat the catalytic surface is limited to the walls of the reactor. Thisdisadvantage is partially resolved by means of optimizedcatalyzer/carrier systems. For the most part, current micro-structurereactors contain small particles or powder, which are incorporated in achannel.

Catalyzer filaments, wires and membranes are also used however (seealso: G. Veser, “Experimental and Theoretical Investigation of H₂Oxidation in a High-Temperature Catalytic Microreactor,” Chem. Eng. Sci.2001, 56, 1265-1273). Metallic nanostructures, particularly those fromtransition metals, are known in heterogenic catalysis due to their highratio of surface area/mass, resulting in lower production costs (seealso: R. Narayanan et al. “Catalysis with Transition Metal Nanoparticlesin Colloidal Solution: Nanoparticle Shape Dependence and Stability,” J.Chem. Phys. B, 2005, 109, 12633-12676).

Originally, research was concentrated on the examination of isotropicmetal particles, and as a result, their catalytic characteristics havebeen studied at length. At present, however, many one-dimensionalnanostructures have been analyzed regarding their use in heterogeniccatalysis. The stabilization of these is a major problem. Theincorporation of nanostructures on a carrier or storage of them inporous matter such as, e.g. Natfion is known from Z. Chen et al.“Supportless Pt and PtPd Nanotubes as Electrocatalysts forOxygen-Reduction Reactions,” Ang. Chem. 2007, 119, p. 4138-4141, whichleads however directly to a decrease in the utilizable catalyzer surfacearea. Furthermore, it must be noted that the catalytic activity isdependent on the distribution of the catalyzer material due to thediffusion processes. Accordingly, the nanoparticles significantlyincrease the surface area/volume ratio, but long-term stability of suchreactors is relatively limited due to the following:

1. Loss of contact between nanoparticles due to corrosion of thecarrier.

2. Dissolving and renewed deposition or Ostwald ripening.

3. Aggregation of the nanoparticles in order to minimize the surfaceenergy.

4. Dissolving of the nanoparticles and migration of the dissolvableions.

Parallel wire and tube structures have already been used as glucosesensors (J. H. Yuan et al., “Highly Ordered Platinum-Nanotubule Arraysfor Amperometric Glucose Sensing,” Adv. Funct. Mater. 2005, 15, 803), aselectrocatalysts, for example, in alcohol oxidation (H. Wang et al.,“Pd. Nanowire Arrays as Electrocatalysts for Ethanol Electrooxidation,”Electrochem. Commun. 2007, 9, 1212-1216) and for hydrogen peroxidereduction (H. M. Zhang et al., “Novel Electrocatalytic Activity inLayered Ni-Cu Nanowire Arrays,” Chem. Commun. 2003, 3022). Nielsch etal. have reported in “Uniform Nickel Deposition into Ordered AluminaPores by Pulsed Electrodeposition,” Adv. Mater. 2000, 12, 582-586, thatpulsed deposition is used for deposition of thin metallic foils.

All in all, there is still a great deal of potential for innovation inthe field of nanotechnology.

GENERAL DESCRIPTION OF THE INVENTION

The invention has the object of providing nanowires, or respectively, ananowire structural element with a large specific surface area.

A further object of the invention is to provide nanowires, orrespectively, a nanowire structural element which may be used in avariety of ways.

The object of the invention is achieved by means of the object of theindependent claims. Advantageous embodiments of the invention aredefined in the dependent claims.

In accordance with the invention, numerous nanowires are produced usinga template based process. For this purpose, a template is preparedhaving numerous nanopores which permeate the template, in particular, atemplate foil, and having a cathode layer on a first side of thetemplate.

For this, a cathode layer, preferably a metallic layer, is depositionedonto a first side of the template foil. The cathode layer can be appliedthrough deposition as a single unit, e.g. through PVD, vaporization orsputtering. Preferably, however, the cathode layer is generated in atleast two layers. For this purpose, a first partial layer isdepositioned, e.g. by means of PVD, sputtering or vaporization, and saidfirst partial layer is then reinforced with a second partial layer bymeans of electrochemical deposition, wherein, as the case may be, adifferent material is used. For example, first a thin metallic layer,e.g. gold, is applied through sputtering, and subsequently this goldlayer is electrochemically reinforced, for example, by a copper layer.This has the advantage that first a relatively thin layer can be appliedthrough sputtering, which is more economical.

Ideally, the template permeated with nanopores, which may be aconventional synthetic foil, in particular a polymer foil, is producedusing high-energy radiation, in particular, irradiation with high-energyions. For example, a polycarbonate foil is irradiated with ions havingan energy of a few to a few hundred MeV/u. For this, the energy of theions is selected such that they fully penetrate the template foil.Accordingly, the energy of the ions is dependent on the thickness of thetemplate foil, and is selected accordingly. High-energy ion beams ofthis type are, for example, available in the accelerator facility of theGesellschaft fur Schwerionenforschung mbH [: Center for Heavy IonResearch; abbreviated: GSI] in Darmstadt. As a result of the irradiationa large number of latent tracks permeate the template foil. The tracksthereby indicate that the molecular structure, i.e. polymer structure ofthe foil is corrupted along the trajectory of each irradiation ion.These tracks are referred to as “latent tracks.” The corruption isgreatest at the core of the track, and is expressed as 1/r². Throughetching, the material having a corrupted molecular structure can beremoved from the track, thus etching the latent track into an openchannel. In this manner, channels can be produced having a diameter of alittle as a few nanometers, and said channels are referred to asnanopores. The latent tracks, and thereby the subsequently generatednanopores are stochastically distributed in relation to the plane of thetemplate surface area.

Subsequently, nanowires are depositioned or grown from anelectroconductive material, in particular, metal, in the nanopores bymeans of electrochemical deposition, wherein the nanowires within thenanopores develop on the cathode layer on a first side of the template.The cathode layer is depositioned onto the template foil prior to thedeposition of the nanowires in the nanopores. This can be carried outprior to the ion irradiation, between the ion irradiation and theetching which generates the nanopores, or after the etching whichgenerates the nanopores.

With this type of nanowire production, the nanopores are filled throughelectrochemical deposition, starting at the inner side of the cathodelayer, wherein the nanowires develop in the nanopores. The developmentprocess of the nanowires starts at the cathode layer, and the nanowiresgrow inside the nanopores from the cathode layer to the opposite side ofthe template foil. For this, the dielectric template foil, which ispermeated with nanopores and electroconductively coated on one side isplaced in an electrochemical deposition device. By means ofelectrochemical deposition of metal ions, the nanowires are depositionedin the nanopores, wherein the metal nanowires within the nanoporesdevelop, in particular, directly on the cathode layer, and are therebyintegrally formed with and thereby firmly secured to the cathode layer.

A process of this sort for the creation of nanowires is basically known,and has been demonstrated, for example, in the “Controlled Fabricationof Poly- and Single-Crystalline Bismuth Nanowires” by T. W. Cornelius etal., Nanotechnology 2005, 16, p. 246-249; and in the dissertation byThomas Walter Cornelius, GSI, 2006; Florian Maurer, GSI, 2007, as wellas by Shafqat Karim, GSI, 2007, which are hereby incorporated asreferences. With this process however, only homogenous nanowires areproduced.

The inventors have found however, that specially structured nanowirescan be produced if the electrochemical deposition is pulsed with theappropriate parameters, more precisely, by using reversed pulsedeposition. With reversed pulse deposition, an alternating sequence ofcathodic deposition pulses and anodic deposition pulses is applied tothe template foil, or respectively, the cathode layer. During thecathodic deposition pulse, a negative voltage occurs at the cathodelayer in relation to the anode, such that during the cathodic depositionpulses the nanowires develop in the nanopores, in each case of a lengthdependent on the duration of the respective cathodic deposition pulsesand a first diameter defined by the diameter of the nanopores. Duringthe cathodic deposition pulse, the nanopores in the correspondingsegment are filled radially in their entirety. Time intervals areprovided between each of the cathodic deposition pulses in which, withreversed pulse deposition, an counter-voltage, or respectively, ananodic counter-pulse is applied. It is clear to the person skilled inthe art that it makes sense to use the terms “cathode layer” and “anode”even though during the counter-pulse the cathode layer is given apositive charge, and the anode, a negative charge. It has been shown,surprisingly, that during the anodic counter-pulse, the development ofthe nanowires is apparently not arrested, but rather, segments of thenanowires with smaller diameters, which do not completely fill thenanopores radially, but rather, are smaller, are generated. Thus, duringthe anodic counter-pulses between the cathodic deposition pulses, thenanowires develop respectively, at a length dependent on the duration ofthe respective anodic counter-pulses and a second diameter in thenanopores, wherein the second diameter is smaller than the firstdiameter. By this means, therefore, segmented nanowires, with analternating series of thicker and thinner segments along the length ofthe nanowires, can be generated. Subsequently, the template foil can bedissolved and removed, using the appropriate solvent with a polymerfoil, thus exposing the segmented nanowires.

It is assumed that the formation of segments is successful when theanodic counter-pulses establish a positive current flow away from thecathode layer. The point at which there is no current flow between thecathode layer and the anode is referred to as equilibrium voltage. Theequilibrium voltage is dependent on, among other factors, material andelectrolyte concentrations, and, where applicable, even dependent ontemperature and can be adjusted by the person skilled in the art for anydeposition system.

The cathodic deposition pulse is understood here to be a voltage pulsewherein voltage in relation to the anode is applied to the cathodelayer, which is negative to a greater degree than the equilibriumvoltage, in order to obtain a positive current flow from the anodetowards the cathode layer.

The anodic counter-pulse is understood here to be a voltage pulsewherein a voltage in relation to the anode is applied to the cathodelayer, which is positive to a greater degree than the equilibriumvoltage, in order to obtain a positive current flow from the cathodelayer towards the anode.

With the deposition parameters used in the embodiment, the equilibriumvoltage of the cathode layer in relation to the anode is approx. −400mV, such that with a voltage of +400 applied to the cathode layer, inrelation to the cathode, an anodic counter-pulse of a relative voltageof +800 mV is obtained relative to the equilibrium voltage.

Preferably, the anodic counter-pulses have a certain positive minimalvoltage relative to the equilibrium voltage in order to obtain thedesired effect. The positive relative voltage to the cathode layerrelative to the equilibrium voltage therefore is preferably at least+100 mV, more preferably at least +400 mV, and particularly preferableis +800 mV ±400 mV. At an equilibrium voltage between the cathode layerand the anode of −400 mV this means there must be an absolute voltagewhich is at least positive to a greater degree than −300 mV, morepreferably positive to a greater degree than 0 mV, and particularlypreferable is +400 mV ±400 mV.

Accordingly, segmented nanowires can be produced using the processdescribed above, wherein first segments with a first diameter and secondsegments with a second diameter alternate, wherein the first diameter isgreater than the second diameter. In other words, there are twoseparated first segments with the larger diameters firmly connected toeach other by a second segment with a smaller diameter located betweenthem. Thus, the first segments shall be referred to as main segments inthe following and the second segments as connecting segments, whereinthe main segments and the connecting segments are made of the samematerial.

The main segments and the connecting segments are integrally connecteddue to the deposition procedure; they form a unified nanowire ofelectrochemically formed material. The main segments are connected toeach other by the connecting segments like pearls on a chain of pearls.

Advantageously, the surface area of the segmented nanowires is largerthan the surface area of homogenous nanowires having a consistentdiameter. Further advantages are specific to the applications.

The specific length of the first segments is determined via the durationperiod of the cathodic deposition pulse. The specific length of thesecond segments is determined via the duration period of the timeintervals. In particular, the length of the first and second segmentscan be determined by the respective durations of the cathodic depositionpulses and the time intervals independently of one another. Accordingly,it is possible to select a predetermined length of the first and secondsegments respectively. For this, the duration of the cathodic depositionpulse and the duration of the time interval are adjusted accordingly inorder to obtain the selected, and thereby predetermined, respectivelengths of the first and second segments.

The first and second segments preferably have different lengths. It ispreferred thereby that the length of the main segment with the largerdiameter is adjusted to be greater than the length of the connectingsegments with the smaller diameter. For this, the duration of the anodiccounter-pulse is selected such that it is shorter than the duration ofthe cathodic deposition pulse. A cathodic deposition pulse durationwhich is less than 60 seconds is preferred, preferably less than 20seconds, and particularly preferred is in the range of 1-5 seconds. Theduration of the anodic counter-pulse is preferably shorter than thecathodic deposition pulse by a factor in the range of 5-1.5. The anodiccounter-pulses preferably have a duration of 0.1-5 seconds, particularlypreferred is 0.3-3 seconds. The duration of the cathodic depositionpulse and the anodic counter-pulse, however, should not be too short toensure the development of the segments. A minimal duration of thecathodic deposition pulse and/or the anodic counter-pulse of at least100 ms is assumed.

The nanowires which are produced consist accordingly ofelectrochemically developed electroconductive material, in particular,of metal or metallic compounds with an alternating sequence of numerousmain segments with a larger diameter and numerous connecting segmentswith a smaller diameter. Thus, a segmented nanowire is referred to here.Because the length of the respective segments can be very small, e.g. inthe range of a few to a few 100 nm, it is possible for a segmentednanowire to consist of more than 100, or as the case may be, more than1,000 alternating pairs of main segments and connecting segments. Inother words, the main segments and the connecting segments alternate inthe lengthwise direction of the nanowire at regular intervals innanometers such that continuously in the lengthwise direction of eachnanowire, between two main segments there is always exactly oneconnecting segment.

Nanowires can be produced using the process of the invention wherein thelength of the main segments is shorter than 100 mn. In general, it ispossible to produce main segments of any length, wherein a length ofless than 1,000 nm seems to be advantageous. The length of theconnecting segments is preferably less than 10 mn, in order to ensuresufficient stability of the segmented nanowires.

The shape of the main segments is basically circularly cylindrical, asthey take the inner form of the nanopores. It has been determined thatthe segmentation is better when the diameter of the nanopores is not toolarge. The diameter of the nanopores and thereby the diameter of themain segments is preferably less than 500 nm, particularly preferred isbetween a few nanometers and a few 100 nanometers. Preferably, thediameter of the main segments remains constant over the length of thenanowire.

If the pulsed voltage is adjusted such that the cathodic depositionpulse and anodic counter-pulse in the deposition device is a regularsequence with consistent pulse durations in each case, then the firstand second segments in at least one section of the length of thenanowire have, in each case, a consistent length, such that thesegmentation of at least a section of the length of the nanowire isconsistent.

In general, with the template based process using a template foil allowsfor numerous nanowires to be produced at the same time. After theirproduction, they can be separated from one another through the removalof the cathode layer such that numerous individual segmented nanowiresare obtained.

It is however also advantageous to produce a stable nanowire structuralelement having an array of many of the segmented nanowires. This can beaccomplished, for example, wherein the cathode layer remains as asubstrate layer of the nanowire array and wherein each nanowire isintegrally formed with the substrate layer. For this, the template foilis dissolved without removing the cathode layer first. In this case, thecathode layer serves a double purpose; on one hand it serves as anelectrode for the electrochemical deposition procedure and on the otherhand is serves as a stable, closed substrate or cover layer for thecompleted nanowire structural element, i.e. it remains as an integralcomponent of the nanowire structural element which is to be produced,and is not removed from said. It is, however, conceivable that after thedeposition of the nanowires in the nanopores, the cathode layer isremoved and a new cover layer is applied.

It is also possible to produce a nanowire structural element thatcontains a nanowire array of segmented nanowires between two coverlayers in such a manner that the nanowire array is enclosed in asandwich-like manner between two cover layers. For this, in addition tothe substrate layer, which is preferably formed by the cathode layer, asecond cover layer is applied to the opposite side.

In order to obtain a stable connection between the nanowire array andthe second cover layer, the electrochemical deposition procedure for thenanowires is carried out at least until caps have formed on thenanowires at the second side of the template foil. In particular, thefollowing two possibilities are proposed for the generation of thesecond cover layer:

The electrochemical deposition procedure is continued after the completefilling of the nanopores, wherein caps are then formed on the nanowireson the second side of the template foil. When the electrochemicaldeposition procedure is continued, the caps grow together to form asurface covering layer and said surface covering layer increases inthickness as the deposition period is increased. Accordingly, one cancontinue the electrochemical deposition procedure wherein the nanowiresare generated or grown until the second cover layer has developedcompletely, forming a sufficiently thick, stable surface covering layer.In this case, the nanowires and the entire second cover layer form aunified structure of electrochemically depositioned material. Hence, thepartial steps (d1) and (d2) in FIG. 1 are carried out as partial stepsof the same electrochemical deposition procedure with the sameelectroconductive material.

Alternatively, the electrochemical deposition procedure for generatingthe nanowires is carried out until caps have developed on the nanowireson the second side of the template foil, and the caps have at least inpart merged together, but a stable second cover layer is not yetgenerated and the procedure is then arrested. The completion of thesecond cover layer is obtained first in a second separate subsequentdeposition procedure, wherein a surface covering additional layer isdepositioned onto the at least partially merged caps, such that thestable second cover layer then consists of the two layered arrangementconsisting of the partially merged caps and the surface coveringadditional layer. The at least partially merged caps form thereby afirst partial layer of the second cover layer, and the additional layerforms a second partial layer of the second cover layer. The separatedeposition procedure can also consist of an electrochemical deposition,but it may also be a PVD process, vaporization or sputtering. Even whenthe separate deposition procedure is an electrochemical deposition, adifferent material may be used for the second partial layer than thatused for the nanowires and the caps. The second partial layer ispreferably electrochemically depositioned differently than the segmentednanowires using a direct current process. In this manner, the depositionperiod of the second cover layer can be reduced.

Accordingly, the second cover layer is partially or entirely generatedthrough electrochemical deposition of an electroconductive material,preferably metal, on the second side of the template foil such that thesecond cover layer is firmly joined to the nanowires by being integrallyformed with the nanowires.

Preferably, the ion irradiation is first carried out and subsequently,but before the etching, the cathode layer is applied. First after thecathode layer is applied to the template foil, the nanopores are etchedfrom the latent ion induced tracks. In particular, therefore, theelectroconductive metallic layer is applied to the template foil andthis is reinforced electrochemically before the latent ion tracks aresubjected to the chemical etching process. In this manner, material fromthe cathode layer being depositioned in the pores is avoided. As aresult, an improved mechanical stability of the nanowire structuralelement generated thereby can be obtained. In addition, the pores arestrictly cylindrical and do not taper at the two ends.

The result of this preferred embodiment is, accordingly, a nanowirestructural element with a hollow chamber-like structure which includesof an array of numerous neighboring segmented nanowires and twoparallel, separated, closed surface cover layers after the template foilis removed. The two cover layers in this embodiment are integralcomponents of the nanowire structural element and are not separated fromthe segmented nanowires, but rather remain integrally joined to said,and more precisely are integrally joined by means of the electrochemicaldeposition procedure at the atomic/molecular level to each other. Thehollow chamber-like structure can be envisioned as a chamber that can beopen at one or more edges.

Accordingly, the nanowires extend perpendicularly between the two coverlayers and the nanowires are integrally joined with their first ends tothe cathode layer and with their second ends to the second cover layersuch that the nanowires firmly connect the two cover layers to eachother, and define a space between the two cover layers. In this manner,a stable sandwich-like nanostructure is formed with a two sided hollowchamber-like structure contained by the cover layers and permeated withthe numerous columns of segmented nanowires running through said.

Furthermore, in this embodiment, that there are interconnected openspaces between the nanowires. The hollow chamber-like structure is,accordingly, open celled on the two-dimensional plane parallel to thetwo cover layers, such that between the two cover layers a fluid can beintroduced in the two-dimensional open cell hollow chamber-likestructure in order to interact with the large surface area of thesegmented nanowires. In other words, a stable, free-standing nanowirestructural element is formed which consists of the two closed coverlayers and the nanowire array which is contained in a sandwich-likemanner between the two cover layers and integrally joined to the same.This nanowire structural element with a nanowire array contained bysurfaces on both sides, or respectively, a layered hollow chamber-likestructure permeated by the nanowire array is ideally suited for use as,for example, a microreactor component, in particular as a microcatalyzercomponent for heterogeneous catalysis.

The distance between the two cover layers, or respectively, the lengthof the segmented nanowires is determined by the thickness of thetemplate foil, and is ideally less than or equal to 200 μm, particularlypreferred is less than or equal to 50 μm. This also applies when thenanowires are separated into single units.

There are further specific structural characteristics of the segmentednanowires generated with the production process however. Because thenanowires are developed from electrochemically depositioned material,they can have a specific crystalline structure, which can, for examplebe examined using X-ray diffraction.

Furthermore, due to the electrochemical deposition, the nanowires of thenanowire structural element are firmly integrally joined at both ends tothe respective cover layers. Because the electrochemical deposition ofthe nanowires is carried out at least until the caps have developed and,if applicable, have merged, the nanowires and at least a portion of thesecond cover layer have merged into a single unit. This too can bestructurally proven, in particular when the nanowires are merged in asingle unit with the caps and said are at least partially mergedtogether. When the deposition process wherein the nanowires aregenerated, after the merging of the caps, has been completed and a firstpartial layer of the second cover layer has been formed thereby and asecond partial layer has been depositioned onto the merged caps in aseparate step with modified process parameters, this can also be provenstructurally. This applies not only when the cover layer consists of twopartial layers of different material.

A larger aspect ratio allows for the generation of a larger activesurface area of the segmented nanowires. The aspect ratio of thenanowires is therefore greater than or equal to 1:50, particularlypreferred is greater than or equal to 1:100.

The surface density of the number of nanowires in a nanowire structuralelement is equally a measure for the active surface area and is ideallygreater than or equal to n/F=10⁷ cm⁻², particularly preferred is greaterthan or equal to n/F=10⁸ cm⁻².

As a specific size for the active surface area of the nanowirestructural element, the geometric specific surface area A_(v) of thenanowires per area of the nanostructure element (surface of the coverlayers) and per the length of the nanowires (height of the hollowchamber-like) may be used. The geometric specific surface area A_(v)should be at least 1 mm²/(cm² μm); preferred however is a larger value,specifically where A_(v) is greater than or equal to 5 mm²/(cm² μm),greater than or equal to 20 mm²/(cm² μm) or even greater than or equalto 100 mm²/(cm² μm) Where applicable, values of up to 1,000 mm²/(cm² μm)may even be obtained.

In the production of the nanowires with the reversed pulse process, thenanowires have a distinct <100> texture, or respectively, a crystallinestructure. With certain metals such as, for example, gold, it may beadvantageous to create the smallest crystallite possible. For this acrystallite size of less than or equal 4 nm is preferred, wherein ingeneral an average crystallite size of less than or equal to 10 nm maybe advantageous.

Due to the crystalline texture, the actual size of the surface area isagain larger than the geometric specific surface area A_(v), which isbased on the smooth cylindrical surface area, ideally by a factor ofaround 4-5.

In the preceding, the production of the template permeated by nanoporesby means of so-called ion beam induced etching is described. It ishowever clear that other processes for the production of a templatepermeated by nanopores may be used, such as, for example, the anodizingof an aluminum foil.

Regarding the production of nanopore arrays in anodic aluminum oxide,reference is made to A. P. Li et al. “Hexagonal Pore Arrays with a50-420 nm INterpore Distance formed by Self-Organization in AnodicAlumina,” Journal of Applied Physics, 84-11, 1998, p. 6023-6026, and areview article by J. W. Diggle, Thomas C. Downie, and C. W. Goulding; p.365-405 DOI: 10.1021/cr60259a005, which are hereby incorporated asreferences. Anodic aluminum oxide templates of this type have theparticular characteristic that the nanopores are evenly arranged in theform of a hexagonal pattern.

A particularly preferred field of application for the nanowirestructural elements is heterogenic catalysis. This means one or morecomponents serve as catalytic components, particularly formicrocatalyzers. For this, it is advantageous to extend a cover layer onone or more of the faces over the edge and allow it to merge with theother cover layer, i.e. the respective edge is integrally connected tothe nanowire structural element. It is particularly simple to firstclose all of the edges and then slice off, for example, two oppositeedges of the nanowire structural element at right angles to the coverlayers.

A microcatalyzer ideally contains a microstructured channel system witha fluid intake and a fluid discharge and at least one nanowirestructural element as a catalyzer element between the fluid intake andthe fluid discharge, in order that fluid may be introduced by means ofthe fluid intake to the hollow chamber-like structure between the twocover layers, fed through the spaces between the nanowires and thenremoved by means of the discharge from the hollow chamber-likestructure. In this manner, the two-dimensional open cell hollowchamber-like structure of the nanowire structural element is formedbetween the two cover layers of the catalytic reaction volumes and thecylindrical surfaces of the nanowire form the catalytically activesurface area which interacts with the fluid within the hollowchamber-like structure. Ideally, due to deposition, the nanowires areformed significantly of (entirely of the same material), for example,platinum, in order that the catalytic element is a fully catalyticelement.

In the following, the invention will be explained in detail using theembodiment examples and in reference to the illustrations, whereinidentical and similar elements have the same reference symbols in partand the characteristics of different embodiments, particularly theprocedures with and without cover layers, can be combined with eachother.

SHORT DESCRIPTION OF THE ILLUSTRATIONS

They show:

FIG. 1 A schematic overview of the production of a nanowire structuralelement.

FIG. 2 A three-dimensional schematic presentation of a nanowirestructural element.

FIG. 3 A schematic overview of the production of a nanowire structuralelement with a three-dimensional (3-D) nanowire network.

FIG. 4 A schematic overview of the production of numerous individualnanowires.

FIG. 5 A three-dimensional presentation of the deposition device usedfor electrochemical deposition.

FIG. 6 A three-dimensional transparent exploded image of the depositiondevice for reinforcing the cathode layer.

FIG. 7 A three-dimensional transparent exploded image of the depositiondevice for deposition of the nanowires and, if applicable, the secondcover layer.

FIG. 8 A detail of the voltage flow of the reversed pulse deposition andan accompanying scanning electron microscope image (SEM) of a segmentednanowire produced thereby.

FIG. 9 The same as FIG. 8 but with a different reversed pulse voltageflow.

FIG. 10 The same as FIGS. 8 and 9, but with yet another reversed pulsevoltage flow.

FIG. 11 A transmission electron microscope image (TEM) of a segmentednanowire.

FIG. 12 An enlarged TEM image of the segmented nanowire from FIG. 11.

FIG. 13 A TEM image of numerous segmented nanowires.

FIG. 14 An enlarged detail from FIG. 13.

FIG. 15 A TEM image of a segmented nanowire.

FIGS. 16 and 17 A TEM image of a segmented nanowire with shorter mainsegments than in FIG. 15.

FIG. 18 An SEM image of a platinum nanowire cap produced using reversepulse deposition.

FIG. 19 An enlargement of a detail from FIG. 18.

FIG. 20 Current flow in the potentiostatic production of a nanowirearray.

FIG. 21 A schematic exploded image of a microreactor with the nanowirestructural element for flow-through operation.

FIG. 22 A schematic presentation of a sensor element with two nanowirestructural elements.

DETAILED DESCRIPTION OF THE INVENTION Example 1—Production of a NanowireStructural Element with Parallel Nanowires

The production of nanowire structural elements is based on a templatebased process. The partial steps of the process are schematicallypresented in FIG. 1 as follows:

(c1) Bombardment of the template foil with ions,

(b) Application of a conductive layer,

(c2) Etching of the ion tracks to form nanopores,

(d1) Deposition of the nanowires and development of the caps,

(d2) Deposition of the second metal layer,

(e) Dissolving of the template.

Preferably the process steps are carried out in the sequence shown inFIG. 1, i.e. (c1), (b), (c2), (d1), (d2), (e). It is however possible touse a different sequence, e.g. to etch from two sides and thensubsequently first apply the cathode layer partial layer ((c2) before(b)). (See, for example, FIG. 3).

With reference to FIG. 1, first a template foil 12 is bombarded withions 14, wherein latent ion tracks 16 are generated in the substance ofthe template foil 12 along the trajectory (c1). The template foil 12 isa polymer foil in this example, specifically, a polycarbonate foil.

Subsequently, on the first side 12 a of the template foil 12, a thin,conductive metallic layer 22 a, e.g. gold, is sputtered onto said,forming a first partial layer. Subsequently, the first partial layer 22a is reinforced electrochemically with a second partial layer 24 a thusforming the first cover layer 26 a, which later serves as an electrodefor nanowire deposition (b). For the electrochemical deposition of thesecond partial layer 24 a, the template foil 12 is mounted in thedeposition device 82 as shown in FIGS. 5-7.

Subsequently, the template foil 12 coated on one side is then removedfrom the deposition device 82, and the latent ion tracks 16 arechemically etched, wherein uniform nanopores 32 are created.Alternatively, the etching process may also be carried out in thedeposition device 82, in that the etching solution is placed in theappropriate cell 88, and after completion of the etching, removed fromsaid. A removal of the template foil and the replacement of said are notnecessary. The diameter of the nanopores 32 can be controlled byadjusting the etching time period (c2).

Following this, the template foil 12 prepared in this manner is placedagain in the deposition device 82, and using the appropriateelectrochemical process, the desired metal is depositioned in thenanopores 32 (d1). When the nanowires 34 reach the ends of the pores 32b at the second side 12 b of the template foil 12, caps 36 begin toform. Under suitable conditions, the caps 36 merge together in a layer,forming a second, closed, but not yet sufficiently stable, metalliclayer 22 b parallel to the first cover layer or cathode layer (d2). Thismetallic layer, in this example, is a first partial layer 22 b, on whicha second metallic layer is depositioned, forming a second partial layer24 b (d2). By means of the second partial layer 24 b, the caps whichhave merged together are embedded in a mechanically stable manner. Inthis way, the first and second partial layers 22 b, 24 b together formthe second cover layer 26 b.

Finally, the polymer foil 12 is dissolved in an organic solvent suitedto this purpose (e). The nanowire structural element 1, produced herebyin accordance with the invention, is shown in FIG. 2. For reasons ofsimplicity, the segmentation of the nanowires is omitted in FIG. 2. Asthe SEM and TEM images show (FIGS. 8-17), the nanowires 34 producedaccording to the invention however, with the correct selection of thedeposition parameters, as will be explained in the following, is in factsegmented. At least the inner side facing the hollow chamber-likestructure 42 of the second cover layer 26 b is at least partially formedhereby by means of an electrochemically depositioned layer 22 b.

The template based method has the advantage that many of the parameterscan be specifically manipulated. The length of the nanowires 34 isdetermined by the thickness of the template 12 used and ideally is10-100 μm, particularly preferred is circa 30 μm±50%. The surfacedensity of the nanowires 34 is determined by the irradiation and forproduction of the array is ideally between 1×10⁷ and 1×10⁹ cm⁻². Thediameter D of the nanowires 34 is determined by the time period of theetching and may be from ca. 20 nm to 2000 nm. The aspect ratio may havevalues of up to 1000.

The thickness of the cathode layer 26 a, and the second cover layer 26 bis controlled through the time period of the respective electrochemicaldeposition, and should be thick enough that sufficient stability isobtained. The thickness of the second cover layer 26 b should be atleast 1 μm. Preferably, the thickness is however greater than 5 μm, e.g.between 5 μm and 10 μm. The same applies to the cathode layer 26 a.

Possible materials for the nanowires are metals which are suited toelectrochemical deposition. Experience has been made with the followingmetals: Cu, Au, Bi, Pt, Ag, Cu, Cu/Co multilayer, Bi₂Te₃.

On the one hand a large number of nanowires 34 with small diameters D isdesired, in order to obtain a large active surface area, and on theother hand a good mechanical stability should be obtained. Theoptimization of this depends on the material used and is adjusted to theneeds accordingly.

For nanowire structural elements 1 with platinum nanowires 34 betweencopper partial layers 24 a, 24 b, a stable construction is produced with10⁸ wires per cm² having a diameter of 250 nm and a length of 30 μm. Theaspect ratio here is 120. Such elements are suited, for example, for useas catalytic elements.

To produce the nanowire structural elements 1, as an alternative topolymer foils 12, other template foils such as hard template foils ofaluminum oxide may also be implemented. The pore diameters which can beobtained here are between 10 and 200 nm. The density hereby issufficient at ca. 6.5×10⁸-1.3×10¹¹ cm⁻². Porous aluminum oxide templatesallow for the generation of uniformly arranged structures. It is alsoconceivable to use templates of ion track etched glasses and mica-films.With these templates, the removal of the template is achieved withhydrofluoric acid (HF), wherein the selection of the metal for the wiredeposition and the metallic layers is somewhat limited.

Example 2—Production of a Nanowire Structural Element with anCross-Linked Nanowire Array

FIG. 3 schematically shows the production of a nanowire structuralelement with an interconnected nanowire array. For this, the templatefoil 12 is irradiated from numerous different angles with ions such thatthe latent tracks and later the intersecting nanopores, or respectively,the intersecting nanowires run at an angle, for example, 90°, to eachother. It is to be understood that other angles are also possible.

For successive irradiation of the template foil 12 at various angles,the template foil 12 is first positioned in a corresponding jet tube ata first angle to the direction of the ion beam, e.g. in the synchrotronof the GSI, and irradiated with a predefined first ion surface density.Subsequently the template foil 12 is tilted in relation to the beamdirection and again irradiated with a predefined second ion surfacedensity. If nanowires are to be generated at more angles, the procedureis repeated for as many angles as desired. To produce a 3-dimensionalnetwork, the template foil 12 positioned at a polar angle to the beamaxis is rotated around the beam axis in the azimuth plane, for exampleFurthermore, the process is carried out as shown in the exampledisplayed in FIG. 1, wherein however, the second cover layer may beomitted.

The nanowire structural element 1 produced in this manner is shownschematically in FIG. 3 (e). The nanowire structural element 1 containsone, or consists of a, nanowire array 35 of intersecting merged togethernanowires 34 which form an integral meshed nanowire network 37. Thenetwork 37 already has a certain inherent stability, due to the meshedstructure of the merged together nanowires, without cover layers, thusbeing open on all sides even though cover layers of the type described,e.g. on one side (substrate layer, formed by the remaining cathode layer26 a) or on two sides forming a sandwich structure, are not ruled out asa possibility.

Example 3—Production of Individual Nanowires

Although it is preferred that a nanowire structural element 1, as isdescribed based on FIG. 1 or FIG. 3, it is however, basically possibleas well, to produce individual segmented nanowires 34. A schematicpresentation of the production steps is shown in FIG. 4. In this case,the electrochemical deposition is arrested before the development ofcaps begins (d1) and subsequently the cathode layer 26 a is removed.This is particularly possible if the cathode layer 26 a or at least thefirst partial layer 22 a consists of a different material than thenanowires 34. The template foil 12 is subsequently dissolved in a step(e) thus causing the individual nanowires 34 to separate (not shown).

Exemplary Parameters for the Production of the Segmenting of theNanowires

All of the examples described in the preceding are produced withsegmented nanowires 34 in accordance with this invention.

For example, a 30 μm thick, a circular shaped (r=1.5 cm) polycarbonatefoil 12 (Macrofol®) irradiated with heavy ions 14 having an energy of11.1 MeV/u and a fluence of 3×10⁷ ions/cm² is used. Prior to theapplication of the conductive metallic layer 22 a, each side of thepolymer foil 12 is irradiated for one hour with UV light, in order toincrease the selectivity of the etching along the tracks 16.

A gold layer 22 a is sputtered onto the first side 12 a of the polymerfoil 12, having a thickness of ca. 30 mn. This is reinforced by apotentiostatic deposition of copper from a CuSO₄ based electrolytesolution (Cupatierbad, Riedel) with a voltage of U=−500 mV, wherein acopper rod electrode serves as the anode (partial layer 24 a). Thedeposition is stopped after 30 minutes, at which point the copper layer24 a is approx. 10 μm thick. Subsequently, etching is carried out fromthe untreated side 12 b of the template foil 12 at 60° C. with an NaOHsolution (6 M) for 25 minutes and thoroughly rinsed with deionizedwater, to remove residual etching solution. At this point, thenanoporous template foil 12 is mounted in the deposition device 82. Thedeposition of nanowires 34 is carried out at 65° C. with alkaline Ptelectrolytes (Pt-OH bath, Metakem).

With reference to FIG. 8, the process of the reversed pulse depositionis used for the generation of the nanowires 34. Unless otherwiseindicated, the voltage indicators refer to the voltage between thecathode layer 36 a and the anode 96 from the perspective of the cathodelayer 36 a.

A cathodic deposition pulse with an absolute voltage of U=−1.3V for 5seconds is followed by an anodic counter-pulse for 1 second with anabsolute voltage of U=+400 mV and so on. The upper illustration shows adetail of the pulsed voltage flow, applied to the cathode layer 26 a,over time. After a few tens of minutes, the deposition is stopped andthe development checked. With the configuration used and a polymer foilas the template foil 12, the equilibrium voltage in this example isapprox. −400 mV, such that the relative voltage of the cathodicdeposition pulse is approx. −900 mV and the relative voltage of theanodic counter-pulse is approx. +800 mV, in each case calculated inrelation to the equilibrium voltage. The alternating cathodic depositionpulse 212 and the anodic counter-pulse 214 are repeated numeroushundreds of times in a deposition period of a few tens of minutes,wherein FIG. 8 shows only a detail of a few pulses, 212, 214.

The segmented nanowire 34 generated with this pulse sequence can be seenin the accompanying SEM image (FIG. 8, bottom). The segmented nanowire34 consists of a periodic alternating sequence of thicker main segments34 c and thinner connecting segments 34 d. The connecting segments 34 dconnect in each case two neighboring main segments 34 c, wherein thenanowire 34 is nonetheless developed from the same material. Theconnecting segments 34 d can also be regarded as periodic contractionsof the nanowire 34. The main segments 34 c have an approx. length of50-100 nm. The connecting segments 34 d have an approx. length of 10 nmor less.

FIG. 9 shows a comparable illustration to FIG. 8, but with cathodicdeposition pulses 212 shortened to 2.5 seconds. Accordingly, the mainsegments 34 c are shorter than in FIG. 8 by half approx. The anodiccounter-pulses 214 are maintained at a constant rate of 1 second.

FIG. 10 shows a comparable illustration to FIGS. 8 and 9, but withcathodic deposition pulses 212 shortened to 1.5 seconds. Accordingly,the main segments 34 c are again shorter than in FIG. 9. It can be seenthat the surface of the nanowire 34 becomes larger when the sequence ofthe segments 34 c, 34 d is shortened and when the nanowire 34 containsmore segments.

With the process in accordance with the invention, it is thereforepossible to set a predetermined length of the rate of repetition of thesegmenting wherein the duration of the cathodic deposition pulse 212 isselected accordingly. In particular, the length of the main segments 34c can be set for a specific desired length. It is therefore assumed thatthe length of the connecting segments 34 d can also be set by means ofselecting the duration of the anodic counter-pulse 214. These lengthsshould not, however, be selected at a size which is too large to obtaina sufficient stability of the nanowires 34. In the FIGS. 8-10 it canfurthermore be seen that the segments 34 c, 34 d within a respectivenanowire have, for the most part, a consistent length along the lengthof the nanowire 34, at least in the illustrated section of the nanowire34. The diameter also remains consistent, which can be attributed to thecylindrical form of the nanopores 32.

If a second cover layer 26 b is to be generated, the deposition iscontinued until the caps 36 have merged sufficiently to the partiallayer 22 b, in order that the potentiostatic deposition of a copperpartial layer 24 b at, for example, U=−500 mV for approx. 30 min. can beapplied.

Finally, the template foil is removed, wherein the entire nanowirestructural element with the template foil 12 is placed in a containerwith 10 ml dichloromethane for several hours. The solvent is replacedthree times in order to fully remove residual polymers.

The inventors assume that the process of the segmenting can be explainedas follows. The prevalent transport process, wherein the metal ions maketheir way to the nanopores 32, is diffusion in the electrolyte solution.For the deposition of the nanowires 34, two different types of diffusionoccur that effect the lengths of the segments. The electrochemicalbehavior of nanoelectrodes which can be observed in the nanowires 34, isdifferent from that of macroelectrodes. The metal ions are reduced onthe electrode surfaces, and are thereby removed from the solution. Inthis manner, a diffusion layer forms and a concentration gradient occursbetween the region without ions and the concentration in the solution.The diffusion layer grows over time in the solution. As a result, thecurrent, limited by diffusion, decreases over time.

For short periods of time, planar diffusion in the nanochannels 32 canbe assumed and the behavior can be described according to the Cottrellequation. This results in a time dependent diffusion limited currentproportional to 1/t^(1/2) wherein t is time. With longer time periods,the diffusion layers reach from the nanopores 32 into the solution,where spherical diffusion behaviors prevail. The current there is, forall practical purposes, independent of time.

In FIG. 20, the voltage flow during a potentiostatic production ofnanowire arrays 35 is illustrated. The curve can be separated into threeregions. In region I, a stronger decrease of the voltage signal can beobserved. Planar diffusion in the nanopores 32 is prevalent here. Inregion II the diffusion layer has reached the solution, andhemispherical diffusion is prevalent. Finally, the nanowires have grownout of the nanopores 32 in region III and formed caps. The electrodesurface increases and planar diffusion occurs again.

The diffusion behavior during the potentiostatic production of nanowirearrays described can be applied to nanowires 34 for the electrochemicaldeposition of arrays with segmented nanowires using reversed pulses, ifthe reversed pulse length is sufficiently short, in order that anexaggerated compensation of the concentration differences does not occurand the diffusion layer does not infiltrate the solution. The pulselengths of the cathodic deposition pulse 212 and the anodiccounter-pulse are accordingly selected in a range which is sufficientlyshort.

If the pulse length of the cathodic deposition pulse 212 and the anodiccounter-pulse 214 is maintained at a constant rate, the segment lengthis proportional to the diffusion current. Because the diffusion currentis relatively constant after a short period, the lengths of the segments34 c, 34 d should remain constant after a short deposition period. Thishas been shown to be true by transmission electron microscope (TEM)images. It can be clearly seen in FIG. 11 that the main segments 34 calong the axis of the wire become longer at first from bottom left totop right until they obtain consistent lengths after approx. 2 μm. A TEMimage in accordance with FIG. 12 from the middle of the wire of the samenanowire 34 displays segments with the same lengths. The illustratedplatinum wire 34 is produced with cathodic deposition pulses of 5seconds at an absolute voltage of U=1.3 V and anodic counter-pulses ofU=0.4 V for 1 second, as shown in FIG. 8.

The relatively shorter segments and increasing lengths along the axis ofthe wire at the beginning of the deposition can be explained in that atfirst the diffusion layer is very short and as a result, only a smallvolume of metal ions are present in the nanochannels 32 when the pulselength of the cathodic deposition pulse 212 can be delivered andreduced. The diffusion layer infiltrates the solution and the number ofelectrochemically active species which enter the diffusion zoneincreases. The diffusion current increases until, due to thehemispherical diffusion at the pore openings, it becomes independent oftime for all practical purposes. At this point, the length of the mainsegments 34 c hardly changes. When a nanowire 34 reaches the end of itsnanopore 32, a hemispherical cap 36 is formed. The neighboring nanowires34 which have not yet reached pore ends grow significantly more slowlybecause nearly all of the substance transport goes to the newlydeveloped cap 36. As the cap 36 grows larger, the planar portion of thediffusion to the cap surface becomes larger and the hemisphericalportion of the diffusion to the cap surface becomes smaller. As aresult, the diffusion current density decreases while the entire currentincreases due to the increasing size of the electrode surface. Thistransition to caps 36 produced with reversed pulse deposition isnoticeable. In FIGS. 18 and 19 a nanowire cap 36 may be observed duringthe development on the side facing the template 12. The ring-likestructures occur as a result of the pulsed process. The cap sectionsgrow outwards from the middle. Each ring segment corresponds to a pairof cathodic deposition pulses 212 and anodic counter-pulses 214. Thesections become thinner at the edges with decreasing diffusion current.In this direction, the entire surface increases with the entire current.Accordingly, the caps 36 are also segmented in rings.

The formation of segments is also ensured through a sufficientlypositive anodic counter-pulse 214. It is assumed that during the anodiccounter-pulse 214 a transport process takes place in the nanopores 32from the end of the developing nanowire to the pore end. This transportprocess is faster on the walls of the nanochannels, wherein a deviationfrom the cylindrical form of the segments occurs, wherein a contractionoccurs respectively forming a thinner connecting segment 34 drespectively. It is assumed that in this case the charge to the walls ofthe pores and the pH value of the electrolyte solution play a role. Theelectrolyte solution is preferably alkaline (pH>7). The segments extendinto the pore “deeper” at the middle than at the edges. This can beobserved in the TEM image (e.g. FIG. 15). It should also be noted herethat there is a clear contraction in each case between the individualmain segments 34 c, resulting in neighboring main segments 34 c beingjoined by connecting segments 34 d, wherein the connecting segments 34 dhave a smaller diameter, which can be clearly seen in FIGS. 13-17.Segmented nanowires of this type are very interesting, because they havea larger surface area than homogeneous cylindrical wires and shoulddisplay a lower conductivity, or respectively, a higher resistance.

By means of the examination of the nanowires 34 segmented in thismanner, the diffusion currents and thereby the diffusion behavior can bereadily examined accordingly. The clear structuring after each pulseallows for statements regarding the temporal progress of the developmentand makes these somewhat one-dimensional nanostructures a model systemfor electrochemical depositions in materials with high aspect ratios inregard to transport processes.

Presumably, the prevailing transport processes play a role in theformation of the segments 34 c, 34 d during the reversed pulsedeposition in the nanochannels 32. It has been shown that for theformation of the segments an alkaline electrolyte solution (pH>7) issuited for the electrochemical deposition of the nanowires 34 incombination with a polymer foil 12, in particular a polycarbonate foil,used as a template foil. The electrolyte solution is preferably highlyalkaline (pH>11).

It is presumed that due to negative surface charges on the polycarbonatetemplates used, as occurs, for example, with glass and quartz surfaces,with a sufficiently positive pH value an electric double layer isformed. The electrostatic forces result in a preferred accumulation ofcations from the electrolyte solution on the surface—a double layer isformed. This consists of a rigid (stellar layer) and a dynamic diffusedborder layer. Like a star, a potential is formed which can be separatedinto two regions. In the rigid border layer, a linear potential decreaseis observed, and in the diffused layer an exponential potential decrease(zeta-potential) is observed. If an electric field is applied along adouble capillary with a double layer, then the dynamic cations in thediffusion layer are drawn in the direction of the cathode. Because thesolvation shell of the ions is carried along and the diffused layerscome quite close, the entire electrolyte solution is moved in thincapillaries. The flow of the entire solution in an electric field isreferred to as the electroosmotic flow (EOF).

In the nanopores, the electric double layer is comparable in itsdimensions to the diameter of the nanopore 32, which is why fluids andions have stronger interactive forces with the walls.

Transport phenomena in nanopores 32 (diameter<1,000 nm or even<500 nm)are distinguished between those in micrometer channels and those inmillimeter channels. Because large regions of the nanopores 32 can becoated with electric double layers which form on the walls, powerfuleffects to the flow rate of the fluid and transport of ions can beexpected in that the flow profile and the spatial distribution of ionsis altered. With very small diameters, such as those present with thenanowires 34 produced here, the flow profile deviates from a flat shape,and becomes parabolic. It becomes increasingly pointed as the diameteris reduced. Therefore, at least some of the parameters:

-   -   Material of the template foil,    -   Relative voltage of the cathodic deposition pulse in relation to        the equilibrium voltage,    -   Relative voltage of the anodic counter-pulse in relation to the        equilibrium voltage,    -   Diameter of the nanopores 32    -   pH value of the electrolyte solution,

are selected such that an electric double layer is formed in thenanopores during the deposition of the nanowires 34 in the nanopores 32,and in particular, such that the dimensions of the electric double layerin the nanopores 32 are in the same size range as the diameter of thenanopores 32.

A parabolic shape can also be seen in the segmented nanowires 34. Themain segments 34 c are only connected to the connecting segments 34 d inthe middle because, due to the parabolic flow profile, the ions firstmake contact with the momentary cathode at this point which isrespectively formed and reduced by the immediately preceding segment.

It is important to have a high pH value to obtain a large zeta-potentialand thereby a large EOF. The pH value of the Pt electrolyte solutionused is approx. pH=13. In addition, the EOF decreases as the electrolyteconcentration decreases. Temperature may also have an effect, as thismay alter the viscosity of the solution.

With reversed pulse deposition, the ion transport for each segment iscarried out anew in the direction of the preceding depositioned segment,and the corresponding profile is formed anew. Because the polarity ofthe relative voltage in relation to the equilibrium voltage is reversedwith each pulse, the transport direction changes with each pulse.

Construction for the Electrochemical Deposition

With reference again to the FIGS. 5-7 the electrochemical deposition ofthe wires 34 in all embodiments is carried out using the depositiondevice 82 which shown in FIG. 5. It consists of a metal housing 84, inwhich the metal sled containing one of the two electrolysis cells 86, 88can be inserted. Due to the good heat transfer properties of metal, itis possible to temper the deposition device by controlled externalheating.

The electrolysis cells 86, 88 made of PCTFE have on their two facingsides, in each case, circular openings 87, 89 of the same size and canbe pressed together firmly with a hand-turned screw. A copper ring 92between the two electrolysis cells 86, 88 serves as a cathode, orrespectively, to establish contact with the first cover layer for theelectrochemical deposition.

With reference to FIG. 6, for electrochemical reinforcement of thepartial layer 22 a, the ion track etched template foil 12 is mountedbetween the two electrolysis cells 86, 88 such that the partial layer 22a, in this case, the sputtered gold layer 22 a, establishes a goodcontact with the ring shaped copper electrode 92. On both sides of thecopper ring being used as a cathode, electrolytes are injected into theelectrolysis cells. The electrochemical reinforcement of the gold layer22 a on the first cover layer 26 a is carried out with a first anode 94,which is placed in the electrolysis cell 86 facing the partial layer 22a, and an external power source with a control device.

After removing the template foil 12 and etching the nanopores 32 outsideof the deposition device 82, the template foil 12 is placed again in thedeposition device 82.

With reference to FIG. 7, the template foil 12 which has been coated onone side and made porous is again placed in the deposition device 82 asin FIG. 6 for electrochemical deposition of the nanowires 34, and whereapplicable, the caps 36 and, where applicable, the completion of thesecond cover layer 26 b, such that the first cover layer 26 a makescontact with the ring electrode 92. At this point, deposition is carriedout on the second side 12 b of the template foil 12 with a second anode96 located in the electrolysis cell 88 on the side away from the firstcover layer 26 a. This deposition procedure is carried out for thegeneration of segmented nanowires 34, as described above, using thereversed pulse process.

Structural Characteristics of the Nanowires

In the framework of the invention the structural characteristics of thenanowires 34 made of different materials is also studied. Withelectrochemically depositioned material it is possible, for example, tocontrol the size of the crystallite. This affects the mechanicalstability, the thermal and electrical transference characteristics aswell as the surface area and thereby also the catalytic activity. Manycharacteristics can thereby be strategically influenced.

In particular, the structure of the nanowires 34 is studied using X-raydiffraction. For this, the texture as a function of the electrochemicaldeposition is analyzed. Should one examine the nanowires 34 producedusing reversed pulse deposition, they display a clear <100> texture,wherein the texture coefficient TC₁₀₀ is 4.16. The crystallites displayaccordingly a preferred orientation, wherein the degree of the alignmentis 83%. An alignment of at least 50% in this case is advantageous. Whereapplicable, the nanowires produced in accordance with the inventiontherefore display a crystallite structure.

Applications

As a catalyzer it is possible to connect a series of numerous nanowirestructural elements 1 according to the invention. Based on measurements,the nanowire structural element 1 is suited individually for applicationin microstructured systems having three-dimensional structures whereinthe internal measurement is less than 1 mm and for the most part liesbetween ten and a few hundred micrometers.

FIG. 21 is a schematic illustration of a microcatalyzer 100, in which ananowire structural element 1 according to the invention is placedbetween a fluid intake 102 and a fluid discharge 104. It is conceivablethat in a microcatalyzer 100 of this sort gas or fluid phase reactionscan be carried out. For this purpose, a gas or fluid flow is directedunder pressure through the microcatalyzer 100.

The nanowire structural element 1 produced according to the inventionwith one or two electroconductive cover layers 26 a, 26 b furthermoreinherently contains an electric contact to all of the nanowiresconnected to the electroconductive cover layer(s) 26 a, 26 b. As aresult, a controlled voltage may be applied to the nanowires 34 therebyenabling electrocatalytic processes. Furthermore, the component may beused as an amperometric sensor.

Production of Microelements using a Radiation Mask

In accordance with the invention, it is possible to create nanowirestructural elements or nanowire arrays of very small sizes, in that thetemplate foil 12, a polymer foil in this example, is irradiated withheavy ions through a corresponding mask. The mask, e.g. a perforatedmask, which is already applied, contains numerous openings orperforations, wherein each opening defines a future microelement. Themask covers the template foil 12 during the irradiation, and latent iontracks 16 are formed thereby, which are subsequently etched to formnanopores 32 in the areas which are not covered by the mask, i.e. at theopenings of the mask. The layout and the shape of the microelement aredetermined therefore by the mask.

This process is particularly suited to the production of many very smallnanowire structural elements, as stated, in the form of microelements.The microelements la which may be produced in this manner consist of twocover layers, firmly joined to the nanowires, which may have a diameterof less than 500 μm, and particularly less than 100 μm, and whereapplicable, even less, to a size of only a few micrometers.

For example, a perforated mask for the ion irradiation withapproximately 2,000 perforations on the entire deposition surface ofapproximately 0.5 cm² is provided, such that approximately 2,000microelements with nanowire arrays can be created as islands in thetemplate foil 12 at one time. After removal of the cathode layer, themicroelements are separated, and break apart into individualmicroelements when the template foil is dissolved and removed.Additional steps may also be carried out however, e.g. in order togenerate cover layers for each individual microelement.

Because all nanowires 34 within each microelement have electricalcontact at both ends, the microelement with nanowire arrays isparticularly suited for production of miniaturized sensors. Due to thelarge number of wires, not only a high sensitivity but also a defecttolerance should result thereby.

The sensor elements may be used for measuring gas flow, temperature andas a motion sensor, for example. With reference to FIG. 22, a sensor 150of this type has at least one measuring device with a first and secondmicroelement nanowire structural element 1 a, wherein the microelementnanowire structural elements la in each case have cover layers 26 a, 26b, wherein each of the two nanowire structural elements 1 a haveelectrical contact through one or both of the two cover layers 26 a, 26b, wherein the two nanowire structural elements 1 a are contactedseparately. A heating element is located between the two microelementnanowire structural elements, such as a microwire 152 which may beheated by means of applying voltage. The calibration of the resistanceof the sensor element 150 is used as a measure for the gas flow rate orthe change in temperature, or change in position.

It is clear to the person skilled in the art that the precedingdescriptions of embodiments are to be understood as exemplary, and thatthe invention is not limited to said, but rather, can be varied innumerous ways, without abandoning the scope of the invention. Inparticular, the production of a microcatalyzer is only one of many usesfor the nanowire structural element of the invention. The segmentednanowires also have many applications as individual units. Furthermore,it is clear that the characteristics, regardless of whether they arepresented in the description, the claims, the illustrations orotherwise, also define significant components of the invention, even ifthey are described in conjunction with other characteristics.

1. A process for the production of nanowires (34), including thefollowing steps: preparation of a template (12) having numerousnanopores (32), which permeate the template (12) from a first side (12a) through to the opposite, second side (12 b) and a cathode layer (26a) on the first side (12 a) of the template (12), growing nanowires (34)in the nanopores (32) by means of electrochemical deposition, whereinthe nanowires (34) grow on the cathode layer (26 a) inside the nanopores(32), wherein the electrochemical deposition is a pulsed deposition,with a temporal alternating sequence of cathodic deposition pulses (212)and time intervals (214) between the cathodic deposition pulses (212),wherein during the cathodic deposition pulses (212), the nanowires (34)develop in the nanopores (32) in each case with a main segment (34 c)having a length which is dependent on the duration of the respectivecathodic deposition pulse (212) and a first diameter defined by thediameter of the nanopores (32), wherein, due to the time intervals (214)between the cathodic deposition pulses (212), in each case a connectingsegment (34 d) is generated on the nanowires (34) in the nanopores (32)with a second diameter, wherein the second diameter is smaller than thefirst diameter, such that segmented nanowires (34) having an alternatingsequence of thicker main segments (34 c) and thinner connecting segments(34 d) along the length of the nanowires (34) are generated, dissolvingand removal of the template (12) to expose the segmented nanowires (34).2. A process according to claim 1, wherein during the time intervalsanodic counter-pulses (214) are applied.
 3. A process according to claim2, wherein the anodic counter-pulses (214) have a relative voltage of atleast +100 mV in relation to the equilibrium voltage.
 4. A processaccording to claim 2, wherein the anodic counter-pulses (214) have alower absolute voltage than the cathodic deposition pulses (212).
 5. Aprocess according to claim 1, wherein the cathodic deposition pulse(212) has a negative relative voltage relative to the equilibriumvoltage of at least 100 mV.
 6. A process according to claim 1, whereinthe cathodic deposition pulse (212) has a negative absolute voltage ofat least 500 mV.
 7. A process according to claim 1, wherein the durationof the time intervals (214) is shorter than the duration of the cathodicdeposition pulse (212).
 8. A process according to claim 1, wherein theduration of the cathodic deposition pulses is shorter than 60 secondsand/or the duration of the time intervals (214) is shorter than 10seconds.
 9. A process according to claim 1, wherein the temporalsequence of cathodic deposition pulses (212) and time intervals (214) isrepeated many times.
 10. A process according to claim 1, wherein thetemplate (12) permeated with nanopores (32) is produced in the followingsteps: (a) preparation of a template foil (12), (b) deposition of thecathode layer (26 a) on the first side (12 a) of the template foil (12),(c1) irradiation of the template foil (12) with an ion beam for thepurpose of generating numerous latent tracks (16) permeating thetemplate foil (12), (c2) etching of the latent tracks (16) to formnanopores (32).
 11. A segmented nanowire (34) which may be produced withthe process according claim
 1. 12. A nanowire of electrochemicallydepositioned material, which includes: an alternating sequence ofnumerous first segments (34 c) having a first diameter and numeroussecond segments (34 d) having a second diameter, wherein the firstdiameter is larger than the second diameter, such that the nanowire (34)in has a segmented structure in the lengthwise direction, wherein thesecond segments (34 d) form connecting pieces between the first segments(34 c).
 13. A nanowire according to claim 12, wherein the first andsecond segments (34 c, 34 d) are connected as a unit and consist of thesame electrochemically depositioned material.
 14. A nanowire accordingto claim 12, wherein the first segments (34 c) with the larger diameterare longer than the second segments (34 d) with the smaller diameter.15. A nanowire according to claim 12, wherein the first segments (34 c)with the larger diameter have a cylindrical shape.
 16. A nanowireaccording to claim 12, wherein the first and/or second segments (34 c,34 d) have in each case a consistent length for at least a portion ofthe length of the nanowire.
 17. A nanowire according to claim 12,wherein the diameter of the first segment (34 c) is smaller than 500 nmover the course of the length of the nanowires.
 18. A nanowire accordingto claim 12, wherein the length of the first segments (34 c) with thelarger diameter is less than 1,000 nm and/or the length of the secondsegments (34 d) with the smaller diameter is less than 50 nm.
 19. Ananowire according to claim 12, wherein the first and second segments(34 c, 34 d) alternate in a regular pattern over the length of thenanowire (34), such that continuously over the length of the nanowire(34) there is always exactly one first segment (34 d) lying between twosecond segments (34 d).
 20. A nanowire structural element that includes:an array (35) of numerous neighboring segmented nanowires (34) accordingto claim 12 and at least one substrate layer (26 a) to which thenanowires, in each case, are firmly joined.
 21. A nanowire structuralelement that includes: an array (35) consisting of numerous neighboringnanowires (34) and two spaced cover layers (26 a, 26 b), wherein thenanowires (34) extend between the two cover layers (26 a, 26 b) and thesegmented nanowires (34) are firmly joineded with their first end (34 a)to the first cover layer (26 a) and with their second end (34 b) to thesecond cover layer (26 b), such that the segmented nanowires (34) firmlyconnect the two cover layers (26 a, 26 b) and define the space betweenthe two cover layer (26 a, 26 b), wherein interconnected open spacesexist between the segmented nanowires (34) such that a stablesandwich-like nanostructure contained on two sides by cover layers (26a, 26 b) and permeated with numerous nanowires (34) in a column-mannerand a two-dimensionally open cell hollow chamber-like structure (42) isdefined in the plane parallel to the cover layers (26 a, 26 b) in such amanner that between the two cover layers (26 a, 26 b) a fluid can be fedthrough the two-dimensional open cell hollow chamber-like structure(42).
 22. A microreactor system that includes: a microstructured channelsystem with a fluid intake and a fluid discharge, at least one nanowirestructural element (1) in accordance with claim 20 or 21 with segmentednanowires (34) as a reactor element between the fluid intake and thefluid discharge, such that fluid from the fluid intake can be introducedto the hollow chamber-like structure (42) between the two cover layers(26 a, 26 b), fed through the open spaces between the segmentednanowires (34) and discharged from the hollow chamber-like structure(42) through the discharge, wherein the two-dimensional open cell hollowchamber-like structure (42) of the nanowire structural element (1)between the two cover layers (26 a, 26 b) forms the reaction volume andthe cylindrical surfaces of the nanowires (34) form the active surfacearea with which the fluid within the hollow chamber-like structure (42)interacts during the flow-through period.
 23. A catalyzer system thatincludes: a microstructured channel system with a fluid intake and afluid discharge, at least one nanowire structural element (1) inaccordance with claim 12 as a catalyzer element between the fluid intakeand the fluid discharge, such that fluid from the fluid intake isintroduced to the hollow chamber-like structure (42) between the twocover layers (26 a, 26 b), fed through the open spaces between thesegmented nanowires (34) and then discharged from the hollowchamber-like structure (42) through the discharge, wherein thetwo-dimensional open cell hollow chamber-like structure (42) of thenanowire structural element (1) between the two cover layers (26 a, 26b) forms the catalytic reaction volume and the cylindrical surfaces ofthe nanowires (34) form the catalytic active surface with which thefluid within the hollow chamber-like structure (42) interacts during theflow-through period.
 24. A sensor element (150), in particular formeasuring gas flow, temperature or motion, containing: at least onemeasuring device with a first nanowire structural element (1, 1 a) and asecond nanowire structural element (1, 1 a), in particular according toany of the preceding claims claim 20, wherein the nanowire structuralelements (1, 1 a) in each case have at least one cover layer (26 a, 26b) joineded to the segmented nanowires (34) for the purpose ofestablishing contact with the respective nanowire structural element andwherein a heating element (152) is located between the nanowirestructural elements.